In Willing, U.S. Pat. No. 3,419,593, in Karstedt, U.S. Pat. No. 3,814,730, and in Ashby and Modic, U.S. Pat. No. 4,288,345, are described complex catalysts comprised of platinum and unsaturated siloxanes, and their use in the so-called hydrosilation reaction between compounds containing silicon-bonded hydrogen and aliphatically unsaturated organic compounds, especially aliphatically unsaturated organopolysiloxane compounds. It is also known to use such catalysts in the reaction of organosilanol compounds with hydrogenosiloxanes to produce a new siloxane and hydrogen gas. Curable compositions useful as encapsulants for electronic components, and the like, comprise organopolysiloxanes having at least two aliphatically unsaturated groups or at least two silicon-bonded hydroxyl groups, a silicon hydride, and such platinum complex catalysts and these can be provided in foamable modifications, as well as filled modifications, containing e.g., from 10 to 300 parts of filler per 100 parts of organopolysiloxane. The disclosures of the above-mentioned patents are incorporated herein by reference, and the latter-mentioned ones as well.
The Willing patent describes, typically, the heating together of symmetrical divinyltetramethyldisiloxane in large excess with chloroplatinic acid then cooling, diluting with still more of the disiloxane, filtering and then washing with water to remove acidity. Adding trace amounts of this composition to a mixture of a polyhydrogenmethylsiloxane and a vinyl-containing polydimethylsiloxane, followed by gentle heating produces a gel, indicating that the known reaction between the .tbd.SiH and CH.sub.2 .dbd.C--linkages has taken place.
Karstedt discloses that superior catalysts can be formed if pains are taken to remove all, or substantially all, of the inorganic halogen which is produced in the reaction between a platinum halide and an unsaturated siloxane. The use of an acid-binding agent like sodium bicarbonate is specified to remove inorganic halogen before the platinum-siloxane complex is used as a catalyst.
Catalysts prepared by the Willing and Karstedt methods have been found to be somewhat less than satisfactory in terms of rate of cure, for example, because they contain numerous intermediate structures and they appear to be encumbered by inhibitory impurities. While both patents appear to recognize the need to remove undesirable materials, such as starting reactants, reaction by-products, etc., neither contemplates the presence, much less the need to avoid anti-catalysts, i.e., the inhibitory impurities. See, for example, inhibitors as described in Nielsen, U.S. Pat. No. 3,383,356.
In Ashby and Modic it is disclosed that two types of olefinic siloxanes of a very specific nature can be used alone, or in combination, as complexes with platinum to produce superior catalysts, e.g., in the reactions of hydrogenosiloxanes with olefinically unsaturated and/or hydroxylated organic compounds and the reactions of hydrogeno-siloxanes with olefinically unsaturated and/or hydroxyl-substituted organosiloxanes.
It has now been found that the prior art procedures can be modified in such a way that valuable complex catalysts are produced which have extreme activity, improved stability and greater ease in production. While the reasons for these beneficial effects are not clearly understood, it is believed that careful selection of component ratios and methods of forming the complex lead to novel complexes different from any described in the prior art, especially the Willing, the Karstedt, and the Ashby and Modic patents mentioned above.
The features which distinguish the present catalysts are process-related. For example, as will be shown hereinafter, a mixture of chloroplatinic acid and divinyltetramethyldisiloxane reacted according to Willing, Example 1, produces a catalyst with a platinum content of 0.27 weight percent, using a ratio of ingredients (before heating) sufficient to provide 0.0037 gram atoms of platinum per mole of vinyl groups in the disiloxane. If, on the other hand, the process of Karstedt, Example 10, is used, the acid-binding agent, sodium bicarbonate being present prior to heating the ingredients, the platinum content is increased to 4.6 weight percent. In curing tests at the same platinum levels, 10 parts per million (ppm) the catalysts show times to cure of 5.0 hours (Willing) and 1.5 hours (Karstedt), respectively. If, on the other hand, the ratio of gram atoms of platinum to moles of vinyl content is changed from Willing's level, up to at least 0.01:1 and preferably more than 0.02:1, and the catalyst forming reaction is conducted in the substantial absence of an acid binding agent, then catalysts with more than 2.0 percent, but less than 4.0 percent by weight of platinum are produced. Most surprisingly, at 10 ppm of contained platinum, the catalyzed compositions cure in only 1.0 hours, showing improved activity over either Willing or Karstedt. The catalysts are better in terms of storage stability, too, maintaining their initial clarity for longer times, with no tendency to precipitate. The catalysts of this invention in addition to having a lower content of inhibiting impurities are industrially more efficient to make, because of the reduction in volumes used, and avoidance of the need to use acid binding agents.